Municipal wastewater is the flow of used water from a community. Although the characteristics of the wastewater vary from location to location depending on various factors, municipal waste water is typically considered as a mixture of storm water and sanitary wastewater. Storm water is runoff from precipitation, and accordingly, reflects the composition of the precipitation and the surfaces with which it comes into contact. Sanitary wastewater is a combination of domestic wastewater and other wastewater such as that discharged form commercial, institutional, and other such facilities. Domestic wastewater includes typical wastes form the kitchen, bathroom and laundry, as well as any other wastes that people may accidentally or intentionally pour down the drain.
The purpose of municipal wastewater treatment is to strip large volumes of wastewater of as much of its harmful elements as possible before releasing it to the receiving environment. Wastewater processing for large municipalities is typically divided into stages: pre-treatment, primary treatment, secondary treatment, tertiary treatment, and additional treatment. Pretreatment comprises screening the municipal wastewater to remove large solid debris such as gravel, wood, plastic and other miscellaneous trash from the wastewater. This step prevents the debris form damaging or interfering with the operation of the downstream equipment. This debris may be disposed of in a landfill or ground up and recycled.
Primary treatment involves encouraging sedimentation by slowing the flow rate of the wastewater and allowing the larger suspended particles to settle naturally due to gravity. This permits floating solids, oils, and greases o be skimmed off the surface. The sedimented contaminants (often referred to as "sludge") may be disposed of in a variety of ways, or "activated" and used as fertilizer. Secondary treatment involves aeration of the wastewater to encourage bacteria and other micro-organisms to grow and consume dissolved organic matter. A second sedimentation step with the addition of a metal salt solution may be employed to remove phosphorus and more of the suspended particles. Chlorine or ozone may also be added to kill harmful micro-organisms. Tertiary treatment employs sand filtration or mechanical filtration to further reduce the suspended particle concentration. Additional treatment such as nutrient removal and dechlorination of the wastewater may be performed subsequent to this step. Although the cleanest water is obtained using the tertiary and additional treatment stages, they are not performed by all wastewater treatment facilities.
In general, screening solid particles from fluid streams by passing a fluid through a screen or sieve having performations of such size the solid particles are retained on the screen surface instead of passing through is well known. In the municipal wastewater pretreatment process however, it is not simply a matter of passing solid particles carried in a fluid stream across a screening device to obtain a separation. The nature of the carried-along debris may often cause blinding of the filter screen. "Blinding" is the clogging of the screens and the reduction of their open area. Further, much of the debris, such as plastic bags, rags, and string, does not lend itself to vibratory screening methods.
Consequently, the debris tends to reduce the capacity of municipal waste separation equipment by clogging the screens and reducing their open area. As more wastewater is received for processing, the debris tends to accumulate in the screening equipment. This obviously further reduces the ability of the screening equipment to screen the wastewater. Eventually, this accumulation threatens the ability of the equipment to effectively screen the wastewater and may even force an overflow or bypass of the pretreatment stage, with detrimental effects on downstream equipment. This undesirable accumulation may result in system shutdown or the need for extra equipment.
Wastewater treatment is essential to the health of the community and the environment. Unless it can be done efficiently, the cost of wastewater treatment may result in higher taxes or increased pollution. Screening equipment which operates efficiently and reliably can reduce the cost of the pre-treatment stage and thereby free up revenue for tertiary treatment or other additional treatment stages which reduce pollution.